1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically operated vehicular transmission having a plurality of speed stages to be established by a selecting operation of a plurality of hydraulic engaging elements, the transmission being coupled to an engine via a fluid torque converter. In this specification, the term "vehicular transmission" means a transmission for a vehicle such as a motor vehicle.
2. Description of the Related Art
In this kind of conventional control apparatus, the following arrangement is employed. Namely, at the time of downshifting, hydraulic pressure of a hydraulic engaging element on the disengaging side is lowered first. The engine is thus raced by the slipping of this engaging element, whereby a rotational speed of an input shaft of the transmission is increased. When an input and output speed ratio (rotational speed of an output shaft/rotational speed of an input shaft) of the transmission has lowered to a synchronous region which is approximately equal to the gear ratio of a speed stage to be established by the downshifting, a hydraulic engaging element on the engaging side to establish that speed stage is engaged so that a smooth downshifting can be performed.
In case a fluid torque converter is interposed between the engine and the transmission, the rotational speed of the engine exceeds the rotational speed of the input shaft of the transmission due to slipping in the fluid torque converter. Especially, when an accelerator is slowly depressed, only the rotational speed of the engine increases by the slipping of the fluid torque converter if the vehicle speed does not change, and a downshifting is sometimes started in this condition. In such a case, if slipping occurs to the hydraulic engaging element on the disengaging side after start of the downshifting, the rotational speed of the input shaft of the transmission rapidly increases so as to approach the rotational speed of the engine. As a result, the input and output speed ratio rapidly decreases and, while the hydraulic pressure in the hydraulic engaging element on the engaging side has not been raised (or boosted) enough, the input and output speed ratio lowers into the synchronous region, whereby the hydraulic engaging element on the engaging side can no longer be engaged at the time of synchronization. According to this, the engine excessively races and shocks occur.
In view of the above point, the present invention has an object of providing a control apparatus for a hydraulically operated vehicular transmission in which a smooth downshifting can be performed even if an excessive increase in the rotational speed of the engine may have occurred due to slipping of the fluid torque converter.